The genera contained in the order Spirochaetales are distinguished by staining reactions, gross morphological characteristics, and the source of the spirochete. The classification scheme has been limited to those characteristics primarily because of the difficulty encountered in the cultivation of the parasitic spirochetes, particularly the anaerobic ones. Consequently little is known of the biologic relationships among the Borrelia, Treponema and Leptospira. Since the aerobic spirochetes, Leptospira, are readily cultivated, considerably more information is available on this group of microorganisms. Members of the genera Borrelia and Treponema which can be cultivated in vitro will be compared to the Leptospira with regard to lipid composition, nutritional requirements, metabolism, immunologic and morphologic characteristics. Later the more nutritionally fastidious spirochetes will be investigated. The treponemes isolated from man will be studied first because many of their nutritional and morphological features are similar to those of the leptospires.